Reverse a String : A Two-Pointer Approach

Introduction: String manipulation is a fundamental operation in programming, and efficiently reversing a string is a common task encountered in various applications. In this tutorial, we’ll explore a powerful two-pointer approach to reverse a string efficiently while achieving O(1) space complexity.

Understanding the Problem: Given a string ‘STR’, our objective is to reverse the order of its characters. For example, if the input string is “abcde”, the output should be “edcba”. We’ll employ a two-pointer approach to achieve this efficiently.

Two-Pointer Approach: The two-pointer approach involves using two pointers to traverse the string simultaneously from both ends towards the middle. We’ll swap the characters at the two pointers until they meet in the middle, effectively reversing the string.

Algorithm Overview:

1. Initialize two pointers, ‘left’ and ‘right’, at the beginning and end of the string, respectively.
2. Swap the characters at the ‘left’ and ‘right’ pointers.
3. Move the ‘left’ pointer one step to the right and the ‘right’ pointer one step to the left.
4. Repeat steps 2-3 until the pointers meet in the middle of the string.

Implementation in Python:

def reverse_string(s):
    s = list(s)  # Convert string to list of characters
    left, right = 0, len(s) - 1  # Initialize pointers

    while left < right:  # Swap characters until pointers meet
        s[left], s[right] = s[right], s[left]  # Swap characters
        left += 1  # Move left pointer
        right -= 1  # Move right pointer

    return ''.join(s)  # Convert list of characters back to string

# Main function
if __name__ == "__main__":
    T = int(input())  # Number of test cases
    for _ in range(T):
        STR = input().strip()  # Read input string
        print(reverse_string(STR))  # Print reversed string
Code language: Python (python)

Time Complexity Analysis:

• The time complexity of the two-pointer approach to reverse a string is O(N), where N is the length of the input string. This is because we iterate through the string once, swapping characters at most N/2 times.

Conclusion: By leveraging the two-pointer approach, we’ve successfully mastered the art of efficiently reversing strings while maintaining O(1) space complexity. This powerful technique is invaluable in various programming scenarios, offering an elegant solution to a common problem. Embrace the simplicity and efficiency of the two-pointer approach in your string manipulation tasks and elevate your programming prowess. Happy coding!